Translation:Dima is a historian.
"The form 'an' is sometimes used before h even when the h is pronounced, but usually only when the first syllable is not accented. The usual example is 'an historic occasion'. Though current in some dialects that pronounce the h, this is considered by many to be affected, pedantic or obsolete."
Normally Duolingo tells him that it's written in cyrillic, if he ever had to write it in Russian. It wouldn't just let him do such mistake.
I think he might have written it wrong, it might be a bug or something. However, the sentence shown by Duolingo doesn't use the same dash and it is placed in a wrong way: "Дима- историк." Maybe he is right that they forgot to add the proper version with a long dash.
EDIT: Well, just had that sentence and it is accepted "Дима – историк". Might be a mistake 6 months ago.
The tasks is not always the same on every sentence. I didn't have to translate this sentence only from Russian to English, but also from English to Russian. The exercise is not static.
So he did well, but the answer was not accepted and he got a mistake. Since then, the issue has been resolved.
And again, if he writes in cyrilic, Duolingo will stop him and tell him he is using a russian keyboard instead of a latin keyboard, so he cannot have a wrong answer if he wrote in cyrillic when the sentence demands english or non cyrillic letters. It will advise him to switch to the latin keyboard.